The HMG-CoA reductase inhibitor having a quinoline base nucleus can be produced by the following process as disclosed in JP-A-1-279866, EP-304063A and U.S. Pat. No. 5,011,930.

A: Reduction of carbonyl groups of (I-6). B: Optical resolution of racemate (I-1). C: Hydrolysis of ester (I-2). D: Step for forming mebaronolactone (I-5) by subjecting free hydroxyl acid (I-4) to a dehydration reaction.
In this process, an unnecessary antipode (I-3) is obtained when ethyl (6E) 3,5-dihydroxyl-7-[2-cyclopropyl-4-(4-fluorophenyl)-quinoline-3-yl]-hepto-6-enoate (I-1) is subjected to optical resolution. Accordingly, it has been an important subject from the viewpoint of the production cost to effectively utilize this unnecessary antipode.
JP-A-08-003138 discloses a process wherein this unnecessary antipode (I-3) is reacted with ozone, and the resulting ozonide (peroxide) is subjected to reducing treatment with dimethyl sulfide to obtain 2-cyclopropyl-4-(4-fluorophenyl)-quinoline-3-carbaldehyde (formula (III)) which can be re-used as an intermediate for the synthesis of the HMG-CoA reductase inhibitor.
However, dimethyl sulfide used in this process has an industrial problem due to a specific unpleasant odor and a handling problem of a low flash point compound. Accordingly, this can not be regarded as an industrially advantageous reaction.
Accordingly, it is an object of the present invention to study a reducing agent of the ozonide obtainable from the compound represented by the formula (I) or (II) and to provide a process for producing 2-cyclopropyl-4-(4-fluorophenyl)-quinoline-3-carbaldehyde industrially advantageously.